Moore was one of the most influential British philosophers of the first half of this century. His reputation in epistemology rests on his defence of commonsense and his point of view, as standardly expounded (cf. Stroud, 1984), is that of the plain man who insists that the validity of our everyday claims to knowledge is not open to serious question, so that philosophers who maintain the contrary can be confidently dismissed.

In truth, Moore’s position was rather more complex. Where the standard account suggests that he simply affirmed the existence of the types of knowledge denied by sceptics, Moore actually developed an intricate anti-sceptical dialectic. One strand of this is his argument from differential certainty (1953, VI, VII). Moore argues that we are entitled to be more certain of the falsity of the conclusions of sceptical arguments than we are of the truth of their premisses. For the premisses will always include general claims about the nature and limits of human knowledge, and the acceptability of such general claims is answerable to their implications concerning particular cases of knowledge; so the conclusions of sceptical arguments undermine our reasons for accepting their premisses. In presenting this argument Moore oversimplifies by suggesting that it just rests on a point about empirical induction; none the less, the argument can be reformulated in the context of an attempt to attain a reflective equilibrium concerning the limits of knowledge, and it brings out a real difficulty sceptics face in arguing for their own position. In my opinion the difficulty is not as decisive as Moore maintains, for sceptical arguments often combine apparently uncontentious theses concerning knowledge with theses from metaphysics and the philosophy of mind that we do not find it easy to abandon. None the less, even though Moore’s argument only establishes a defeasible presumption against scepticism, it is not just the question-begging contraposition that the standard account of his position represents him as propounding.

Another strand in Moore’s anti-sceptical dialectic is the charge that the sceptic’s presentation of his position is incoherent since the sceptic cannot argue for his conclusion without representing himself as having knowledge of his premisses and of their implications. Moore’s charge rests on the thesis that assertion essentially involves a claim to knowledge (1959, p. 248), which is the epistemic analogue of the principle underlying Moore’s paradox (see MOORE’S PARADOX) and is suggested by the problematic nature of statements of the form “p but I do not know whether p”. In my view, however, the possibility of making hedged assertions, of the form “p I think,” which manifestly do not involve a claim to knowledge, undermines this argument. For this possibility strongly suggests that the claims to knowledge implied by normal assertions are just conversational implicatures, in Grice’s sense (see GRICE), which can be cancelled by hedging when the situation requires. Furthermore, even if Moore’s thesis about assertion is accepted, the sceptic can just present his argument as a series of hedged assertions and thereby avoid the incoherence Moore imputes to him.

The writings in which Moore seems to live up to the standard account of him are his 1925 paper “A Defence of Common Sense” and his 1939 lecture “Proof of an External World” (both in Moore, 1959). For in the first paper he sets out, without any apparent argument, a list of commonsense truisms of which he maintains that he has certain knowledge; and in the second he famously maintains that it would be absurd to question his knowledge of such things as that his hands are before him. Yet I think that in both cases the standard account misunderstands Moore’s purport. The “Defence” was originally written in response to an invitation to provide a “personal statement” of his philosophical position; so Moore’s initial affirmations of common sense knowledge are just statements of his position, not attempts to establish its validity, though Moore does also attempt here to refute sceptical theses, which he attacks as incoherent. In the “Proof” Moore aimed to prove only the existence of an external world, not the existence of knowledge of such a world – i.e. to refute idealism (see IDEALISM), not scepticism. This distinction may seem tenuous: if Moore’s proof succeeds, then, in giving it, does he not also prove the existence of his own knowledge of an external world? Yet that was not what Moore felt; in his view, a proof of knowledge that p requires the refutation of sceptical arguments, but such a refutation is neither required for, nor accomplished by, a proof that p itself. Admittedly this latter proof requires premisses that are in fact known, but Moore does not think he needs to prove the existence of this knowledge – it will suffice to remind his audience of the kinds of thing we all take it for granted that we know (cf. Moore, 1942, pp. 668–9, for an unequivocal repudiation of the thought that sceptical arguments can be refuted by the dialectical strategy employed in the “Proof”.

What, then, is Moore’s contribution to epistemology? If he was not the philosopher’s idealized plain man but just another plain philosopher unsuccessfully defending commonsense against sceptical arguments (cf. “Certainty” in Moore, 1959), wherein lies the value of his writings? In my view it does lie in his attempted defence of common sense, but that defence needs to be set in the context of a naturalistic epistemology (see NATURALIZED EPISTEMOLOGY) which Moore himself never adumbrated. In this context Moore’s affirmations of certainty concerning particular matters of fact (“Here is one hand,” etc.) signal that involuntary commitment to the existence of our perceptible environment on which a naturalistic epistemology relies in order to get itself started. And Moore’s remarks about the “strangeness” of our epistemological situation vis-à-vis his commonsense truisms (1959, p. 44) reflect the fact that a naturalistic epistemology just incorporates our general presumptions about the structure of the world and our relationship to it rather than providing us with an independent method for verifying them. This last line of thought was famously developed by Wittgenstein in On Certainty, particularly in his remarks concerning “Moorean propositions” (which correspond to Moore’s truisms) (see WITTGENSTEIN). So my judgement is that it was Wittgenstein who brought to fruition the potential of Moore’s epistemology.

See also COMMONSENSISM AND CRITICAL COGNITIVISM; SCEPTICISM, CONTEMPORARY.